A mold powder and an immersion nozzle are popularly used in continuously casting molten steel.
For example, a mold powder comprising 35.46 wt.% SiO.sub.2, 6.08 wt.% Al.sub.2 O.sub.3, 36.87 wt.% CaO, 8.05 wt.% Na.sub.2 O, 5.33 wt.% ignition loss and impurities is added onto the meniscus of molten steel in a mold. The mold powder is melted into a vitreous state by heat from the molten steel to cover the molten steel meniscus, and at the same time, penetrates into gaps between sides of solidified steel and the mold inner walls to cover the surface of cast strand. The molten steel and the cast strand are thus isolated from air and protected from oxidation. Furthermore, the molten mold powder layer absorbs non-metallic inclusions floating up on the molten steel meniscus.
On the other hand, an immersion nozzle is attached as a protrusion substantially vertically to the bottom of a tundish, and the lower portion thereof is immersed into the molten steel in the mold across the above-mentioned molten mold powder layer. The molten steel in the tundish flows down through the immersion nozzle and is teemed into the mold without being exposed to air except during the initial stage of teeming.
By using an immersion nozzle together with a mold powder, therefore, it is possible to effectively prevent such inconveniences as oxidation of molten steel in the mold and the cast strand extracted from the mold, occurrence of turbulence in the molten steel, entanglement of air, mold powder and slag, and molten steel splash, thereby giving a sound cast strand excellent in surface quality as well as in inner quality.
Amorphous silica, zircon-graphite and aluminagraphite refractories are known as materials for the abovementioned immersion nozzle, and an immersion nozzle is manufactured by forming any of these refractories into a shape for example as shown in the schematic sectional view of FIG. 1, and firing said formed body. In FIG. 1, 2 is a nozzle body, 3 is a collar portion, 7 is a bore, and 8 is an exit port. Molten steel in a tundish is teemed into a mold through the collar portion 3, the bore 7 and the exit port 8 of the immersion nozzle. The immersion nozzle, through the bore 7 of which high-temperature molten steel flows down, is exposed to radical temperature change and thermal shock particularly in the initial stage of teeming, and in addition, the bore 7 is eroded by molten steel. Furthermore, the portion of the outer surface of the nozzle body 2 in contact with the molten mold powder layer is most seriously eroded by molten steel and molten mold powder. Along with the recent trend toward larger continuous casters, molten steel of more than five batches of ladle is often continuously teemed for casting.
An immersion nozzle is therefore required to have various properties to meet the aforementioned severe service conditions. Among these properties, those which have the most important effect on the service life and should therefore be satisfied include spalling resistance in the initial stage of molten steel teeming, erosion resistance against molten steel and erosion resistance against molten mold powder. A nozzle which does not satisfy these three properties at the same time cannot withstand continuous teeming of molten steel of more than five batches of ladle into a mold.
However, all the above-mentioned amorphous silica, zirconia-graphite and alumina-graphite refractories have respective mertis and demerits, and it is very difficult to manufacture an immersion nozzle capable of withstanding the above-mentioned severe service conditions from a single kind of refractory selected from those mentioned above. More specifically, the amorphous silica refractory has a very small thermal expansion and a relatively satisfactory erosion resistance against molten mold powder as from 2 to 3 mm per cycle of continuous casting of molten steel of a batch of ladle. On the contrary, however, the amorphous silica refractory is susceptible to spalling because of transformation of amorphous silica during service for a long period of time, and has a relatively low erosion resistance against molten steel, particularly high-Mn molten steel. The zircon-graphite refractory has a relatively satisfactory erosion resistance against molten steel, while erosion resistance thereof against molten mold powder is problematic. The alumina-graphite refractory has a good erosion resistance against molten steel. The zircongraphite and alumina-graphite refractories, both containing graphite, have a high thermal conductivity and hence are capable of well withstanding radical temperature change and thermal shock. In contrast, however, the structure becomes porous as a result of oxidation and/or dissolution into molten steel of graphite, and erosion is caused by molten steel and molten mold powder penetrating into portions thus becoming porous, this forming a drawback common to these refractories.
With a view to solving the aforementioned problems and thus improving erosion resistance against molten steel and erosion resistance against molten mold powder as required for an immersion nozzle, the following immersion nozzles are proposed:
As shown in the schematic sectional view given in FIG. 2, a highly erosion-resistant refractory layer made of a material such as zirconia refractory is arranged on at least one of the inner surface 10 of a bore 7 and an exit port 8 of a nozzle body 2 mainly comprising amorphous silica and the outside portion 1' of said nozzle body 2 in contact with a molten mold powder layer, so as to be flush with the inner and outer surfaces of said nozzle body 2; and zirconia, silica-zirconia, zirconia-mullite, mullite and chromium oxide refractories are suitable as said highly erosion-resistant refractory (hereinafter referred to as the "prior art (1)").
In the prior art (1), the nozzle body 2 comprises mainly amorphous silica. Amorphous silica has a very small thermal expansion and a relatively satisfactory erosion resistance against molten mold powder, as mentioned above, while having a relatively low erosion resistance against molten steel.
Also in the prior art (1), zirconia (ZrO.sub.2) which is the main material of the refractory layer(s) arranged on the portion indicated by 10 in the bore 7 of the nozzle body 2 and/or on the portion indicated by 1' on the outside portion of the nozzle body 2 has an excellent erosion resistance against molten mold powder. However, in firing, there occurs a considerable difference in thermal expansion between the arranged refractory layer(s) 10 and/or 1' having a high thermal expansion and the nozzle body 2 comprising amorphous silica having a very small thermal expansion, and as a result, spalling may be caused in both the nozzle body 2 and the portion(s) 10 and/or 1'. It is thus difficult to obtain an immersion nozzle free from a defect.
It is therefore difficult for the immersion nozzle of the prior art (1) to withstand severe service conditions including continuous casting of molten steel of more than five batches of ladle.
A refractory layer excellent in erosion resistance against molten mold powder is arranged on the entire surface of the nozzle body comprising a refractory excellent in erosion resistance against molten steel or on the outside portion thereof in contact with a molten mold powder layer, so as to be flush with the outer surface of said nozzle body, or, to form a protrusion from the outer surface of said nozzle body; alumina-graphite refractory is suitable as said refractory excellent in erosion resistance against molten steel, and amorphous silica refractory is suitable as said refractory excellent in erosion resistance against molten mold powder (hereinafter to as the "prior art (2)").
In the prior art (2), the nozzle body comprises mainly alumina-graphite as in the present invention described later. As mentioned above, alumina-graphite is excellent in erosion resistance against molten steel.
Also, in the prior art (2), the main material for the refractory layer arranged on the outside portion of the nozzle body is amorphous silica. As described above, amorphous silica has a relatively satisfactory erosion resistance against molten mold powder, while having a low erosion resistance against molten steel, especially high-Mn molten steel. In addition, amorphous silica is susceptible to spalling because of transformation of amorphous silica during service for a long period of time.
Therefore, the immersion nozzle of the prior art (2) obtained by arranging an amorphous silica refractory layer having a low erosion resistance against molten steel on the outside portion of the nozzle body in contact with molten mold powder layer is problematic in erosion resistance against molten steel and it is difficult for such an immersion nozzle to continuously cast molten steel of more than five batches of ladle.
As shown in the schematic sectional view given in FIG. 3, the portion 1" of a nozzle body 2 susceptible to local erosion in contact with a molten mold powder layer, or said portion 1" and a portion 1'" immediately therebelow (i.e., 1"+1'") are formed with a zirconia-graphite refractory or an MgO.Al.sub.2 O.sub.3 spinel-graphite refractory; and the remaining portion of said nozzle body 2 is formed with an alumina-graphite refractory; said zirconia-graphite refractory having preferably the following chemical composition:
and said MgO.Al.sub.2 O.sub.3 spinel-graphite refractory having preferably the following chemical composition:
In FIG. 3, 7 is a bore, and 8 is an exit port. The immersion nozzle of the prior art (3) is common with the immersion nozzle of the present invention described later in that the refractory used for the portion 1" of the nozzle body in contact with a molten mold powder layer and the portion 1'" immediately therebelow consists essentially of carbon (C), zirconia (ZrO.sub.2) and silica (SiO.sub.2). However, in the prior art (3), the carbon content is as high as from 15 to 30 wt.%. Consequently, the structure of the refractory becomes porous as a result of oxidation and/or dissolution into molten steel of carbon, and molten steel and molten mold powder penetrating into portions thus becoming porous causes erosion of zirconia, thus accelerating erosion of the portion of the immersion nozzle in contact with molten mold powder layer.
It is therefore difficult for the immersion nozzle of the prior art (3) also to withstand severe service conditions including continuous casting of molten steel of more than five batches of ladle.